Field of the Invention
The present invention relates to a robot apparatus which stops a robot when a stop order is received in the middle of force control, a robot controlling method which is performed by the robot apparatus, a program which is used for the robot controlling method, a recording medium which stores or records therein the program, and an assembly manufacturing method in which the robot apparatus is used.
Description of the Related Art
In general, a robot is controlled mainly by either position control or force control. In the position control, when an operation by which the robot comes into contact with a workpiece (work) is performed, a load (burden) is added on the workpiece and/or the robot if there is a position error or the like in the workpiece. Thus, it becomes hard or difficult to perform the operation.
For this reason, position-based force control (admittance control) is performed as the force control. In the position-based force control, a position instruction value by which the robot follows the workpiece is calculated based on the value of a force sensor provided at the tip (end) of the robot. Incidentally, when a stop order is received in the middle of the position-based force control, the force control is switched to the position control, and then the robot is decelerated and stopped (see Japanese Patent No. 5371882).
However, since the position control is used as a minor loop in the position-based force control, responsiveness of the position-based force control is necessarily lower than responsiveness of the position control. In general, a response frequency of the position control is restricted by the natural vibration frequency of the robot. In particular, when the robot is in contact with the workpiece, it is difficult to increase the responsiveness because of a problem of safety.
For this reason, a robot which can achieve high-responsiveness force control is currently developed. More specifically, torque-based force control in which the force control is performed based on a torque instruction value is developed. In the torque-based force control, it is possible to increase the responsiveness without being restricted by the natural vibration frequency of the robot.
However, since the robot has an individual difference which is caused by an error or errors occurred in the process of manufacturing the parts thereof, a positioning error occurs in the robot due to the individual difference. Here, it is difficult to accurately grasp or know the individual difference for each robot. Therefore, when the robot is operated by the position control, the operated robot is resultingly moved to a position which is slightly deviated from a target position.
For this reason, in the high-responsiveness torque-based force control, when the control state is switched from the force control to the position control and then the robot is stopped as described in Japanese Patent No. 5371882, the robot is stopped at the position which is deviated from the target position in the position control due to the individual difference of the robot. Therefore, in a case where the robot is in contact with the workpiece or the workpiece grasped by the robot is in contact with another workpiece, there is a problem that an excessive load is added on both the robot and the workpieces.
Besides, the responsiveness is high in the torque-based force control. Therefore, if a moving speed of the robot is high, it takes time before the robot is decelerated and stopped when a stop order is received. For this reason, there is a possibility that the robot cannot stop at the stop target position and resultingly overshoots it. When the robot overshoots the stop target position, as well as the above, there is a problem that an excessive load is added on the robot and/or the workpiece.
For example, in a case where a mating (fitting) operation such as peg-in-hole is performed using the torque-based force control, if decelerating and stopping the robot by the position control, overshoot occurs because of the position deviation (aberration) due to the individual difference and the high responsiveness. For this reason, there is a case where excessive force is added to the workpiece and thus the workpiece is caught by or hitched on a matching (fitting) hole. Since it is difficult to release or remove the workpiece caught by the matching hole, there is a problem that it takes time before such a troublesome situation is solved. In addition to the matching operation, if the robot is decelerated and stopped by the position control while a pushing operation of pushing the workpiece by certain force is being performed, the workpiece is resultingly pushed by excessive force. Thus, as well as the above, there is a problem that an excessive load is added on the robot and/or the workpiece.